758 BELL SYSTEM TECHNICAL JOURNAL 



Except at low frequencies, where the internal inductance of the iron 

 wire is large, the total inductance of an iron-wire circuit is not far differ- 

 ent from that of the corresponding copper circuit. The capacitance 

 and leakage conductance of iron-wire circuits are substantially the 

 same as for similar pairs of copper wire. 



Typical attenuation curves for several gages of BB iron wire in dry 

 weather are shown in Fig. 20. These curves are based upon experi- 

 mental results. It will be noted that the attenuation of an iron-wire 

 circuit averages about ten times that of the corresponding copper-wire 

 circuit. It should be understood that the attenuation values for an 

 iron-wire circuit are subject to rather wide variations in practise, partic- 

 ularly because of the effects of corrosion. There is some change in the 

 attenuation of an iron-wire circuit with change in weather conditions, 

 but this is a relatively small percentage. The impedance of an iron- 

 wire circuit has the same order of magnitude as the impedance of a 

 similar copper-wire circuit. 



Appendix I 



Temperature Coefficient of Resistance at High Frequencies 



In the skin effect literature cited in the text, it is shown that at high 

 frequencies the a.-c. resistance R is 



R = 0.00979 V/i^o, (15) 



where Rq is given by Equation (6) and / is the frequency. Differen- 

 tiating with respect to t, and substituting, we find that 



1 dR 1 \R^i 



^^ = ^^=rW^ ^^^^ 



and for ordinary ranges of temperature 



^, = f • (17) 



Appendix II 



Condition for Minimum Attenuation 



The attenuation at high frequencies is given by Equation (7). 



Differentiating this with respect to ylj , and setting the result equal to 



